1. Field of the Invention
The present invention generally relates to a communication system, and more particularly to a system and a method for transmitting/receiving neighbor Base Station (BS) information in a communication system.
2. Description of the Related Art
In the next generation communication systems, active research is taking place in order to provide users with high-speed services having various Qualities of Service (QoSs). Especially, in a Broadband Wireless Access (BWA) communication system, which is one of the next generation communication systems, active research is being conducted in order to support a high speed service capable of guaranteeing mobility and QoS. Representatives of such communication systems include an Institute of Electrical and Electronics Engineers (IEEE) 802.16e communication system.
An IEEE 802.16e communication system, which is one of the BWA communication systems, considers not only immobility of a subscriber station but also a mobility of the subscriber station. Therefore, when a Mobile Station (MS) having both immobility and mobility characteristics moves between multiple cells in the IEEE 802.16e communication system, the IEEE 802.16e communication system can guarantee the mobility of the MS by supporting handover of the MS.
In an IEEE 802.16e communication system, in order to support handover of an MS, a Base Station (BS) transmits information of each neighbor BS controlling a neighbor cell, which neighbors the cell (i.e. serving cell) controlled by itself, to all MSs within the serving cell. Specifically, the BS periodically broadcasts a mobile neighbor advertisement (MOB_NBR-ADV) message including information of neighbor BSs, thereby previously reporting state information of the neighbor cells to corresponding MSs. That is, when an MS located within one cell moves to a neighbor cell, the MS previously receives a MOB_NBR-ADV message broadcast by a BS (serving BS) controlling the cell in which the MS itself is located. As a result, the MS can perform handover after previously acquiring information (e.g. network identifier (ID), channel characteristics, etc.) of the BS (neighbor BS) controlling the neighbor cell to which the MS will move.
As described above, an IEEE 802.16e communication system supports handover of an MS. However, in an IEEE 802.16e communication system, when an MS fails to receive a MOB_NBR-ADV message periodically broadcast by a serving BS, the MS may fail to perform normal handover.
Specifically, the neighbor BS information is periodically fragmented and broadcast with a Broadcast Connection Identifier (BCID) through the MOB_NBR-ADV message. At this time, the MS can perform normal handover only after receiving all fragments of the MOB_NBR-ADV message. Therefore, if the MS fails to receive all fragments of the MOB_NBR-ADV message due to the communication environment or system situation, it may be impossible for the MS to perform normal handover. Further, if the period of the MOB_NBR-ADV message has passed and the MS defers the handover until it receives all fragments of the MOB_NBR-ADV message, delay may occur and connection may be interrupted during the handover.
Meanwhile, since the MOB_NBR-ADV message is broadcast with a BCID as described above, data bursts having the BCID are encoded according to a most robust encoding scheme so all MSs within the cell can receive the data bursts. For example, data bursts having the BCID are encoded according to a Quadrature Phase Shift Keying (QPSK) ½ scheme, which is the most robust encoding scheme in the IEEE 802.16e system, and are broadcast according to a six time repetition technique. Therefore, the size of the MOB_NBR-ADV message broadcast within one frame is physically restricted. That is to say, the MOB_NBR-ADV message is configured to express a difference between physical or logical shapes of the serving BS and the neighbor BSs. Therefore, the larger the difference between the serving BS and the neighbor BSs, the larger the size of the MOB_NBR-ADV message, which causes a limit in configuration of the MOB_NBR-ADV message.
The MOB_NBR-ADV message may be fragmented into a maximum of 16 fragments, each of which may contain information on a maximum of 256 neighbor BSs. However, when the MOB_NBR-ADV message is encoded according to a most robust scheme as described above, each fragment of the MOB_NBR-ADV message can include information on less than 10 neighbor BSs, and it is thus impossible to broadcast information on all neighbor BSs to MSs within the serving cell.
Further, since the BCID is used in broadcasting the MOB_NBR-ADV message as described above, all MSs within the serving cell can acquire the information on all the BSs included in the MOB_NBR-ADV message. However, if the serving BS wants to send information on a particular neighbor BS to only a particular MS, such a job cannot be performed by the MOB_NBR-ADV message, which is a broadcast message. Further, since the MOB_NBR-ADV message is a broadcast message, a message authentication code for which cannot be used, an MS having received the MOB_NBR-ADV message cannot perform an authentication job with a BS having broadcast the MOB_NBR-ADV message. Further, the IEEE 802.16e system defines 114 downlink preamble indexes within the same frequency. When the MOB_NBR-ADV message is broadcast based on the BCID, the preamble index is unique in the MOB_NBR-ADV message. Therefore, neighbor BSs of a particular BS cannot reuse the downlink preamble index within the same frequency.